An A-C Kelvin bridge for the audio-frequency range

THE present trend in electrical measurements indicates the need for a test method by which the ratio and phase angle of current transformers can be measured at adiuo frequencies with the same accuracy as they are now measured at power frequencies. The theoretical aspects of several test methods are being investigated at the National Bureau of Standards. It is essential to this study to determine the extent to which the present test equipment,1 comprising 4-terminal impedors and a mutual inductor in a potentiometer network, can be utilized in high-frequency measurements. As part of this program, an a-c Kelvin bridge has been developed for measuring the characteristics of the 4-terminal impedors at audio frequencies. At present the bridge method permits resistance and phase-angle measurements up to 6,000 cycles per second (cps). A substitution technique is employed in which the bridge is balanced first with the unknown and then with a primary standard having a computable skin effect and phase angle. The resistance of the computable standard is previously adjusted and accurately measured in terms of a d-c standard resistor. The range of adjustment for the standard extends from 0.1 to 0.05 ohm. With additional computable standards now under study, and minor modifications of the circuit, measurements can probably be extended to a minimum resistance of 0.001 ohm and a maximum frequency of 104 cps.